The proposed research is concerned with the mechanisms underlying photoreceptor cell response in the vertebrate retina. Intracellular recordings will be made from photoreceptor cells of the retina of the mudpuppy, Necutuus maculosus. During recording, the retina will be treated with calcium, cyclic nucleotides and drugs which specifically affect their metabolism. The specific objectives are: 1. to determine the role of calcium in photoreceptor response and adaptation; 2. to determine the role of cyclic nucleotides in photoreceptor response and adaptation; 3. to determine the possible interactions between calcium and cyclic nucleotides in photoreceptor response and adaptation.